1. Field of the Invention
The present invention relates generally to strain-gage sensors. More particularly, the invention is concerned with sensors that are mounted to dynamic load-bearing structures, such as sucker-rod type oil well pumps, to produce an electrical output proportional to the deflection of such structures.
2. Discussion of the Invention
Oil well pumping units sometimes require a system to accurately determine fluid pounds in the well. This information is primarily required on wells that pump off, that is wells that do not contain enough fluid to permit them to be pumped continuously. A system of pump-off control, called "P.O.C." is used on these wells. A P.O.C. consists of either a load cell installed on the pumping rod, or a sensor mounted on the walking beam which detects deflection or load while the pump is operating. Typically the sensor interfaces with the controller to produce information for well analysis. Such systems not only provide information concerning the amount of fluid in the well, but also shut the pump down when the well is empty, thereby avoiding excess wear on the pumping equipment and saving energy. The P.O.C. system also protects the well if there is a malfunction in the sucker rod string or drive train by shutting the pump down when it detects such malfunctions.
A device for use in measuring strain in the polish rod of a rod-pumped well unit is disclosed in U.S. Pat. No. 3,965,736 issued to Welten, et al. Discussed in this patent is a system utilizing a strain-gage transducer welded to the top flange of the walking beam of an oil well pumping unit. In systems of this type, the sensor is welded to the walking beam in order to achieve maximum sensitivity. However, because the apparatus of the present invention is five to ten times more sensitive than that of the prior art welded systems, the sensor can be affixed to the load bearing surface of the walking beam or at any convenient location using conventional clamps. The advantage of clamps over welding are many, but especially advantageous is the ease of installation and removal of the sensor for repair or replacement. The extra sensitivity provided by the apparatus of the present invention also enables it to be used on very "light" or shallow wells where less sensitive sensors have not been effective.
Another major drawback of the prior art strain-gage beam sensors is their inability to compensate for temperature induced strain on the structure. Infrared radiation from the sun is especially troublesome because it causes uneven heating and radical expansion of the steel structure, or walking beam. Attempts have been made to solve this problem by painting the sensor black and by trying to closely match the material of the sensor with that of the structure to attempt to achieve the same thermal coefficient of expansion. However, such attempts have been generally unsuccessful. The present invention uniquely overcomes this problem by providing a novel, active electronic temperature correcting circuit.